Low platelet counts can be caused by many bacterial, viral, fungal and parasitic infections, including ehrlichiosis, a tick-borne disease. Canine distemper virus is another common cause of low platelet counts.
Low platelet counts are usually caused by a condition called immune-mediated thrombocytopenia, or IMT. It is an autoimmune disorder, which means that the body's immune system attacks its own blood platelets. It can occasionally occur for no obvious reason and is also linked to noninfectious diseases such as cancer or as a reaction to certain drugs. There are occasional cases of low platelet counts that are not caused by IMT, but these are rare and usually related to bone-marrow diseases. Some breeds, including the Great Pyrenees, may have low platelet counts due to congenital issues.
Tick-borne diseases are one of the most common causes of IMT. In addition to ehrlichiosis, dogs can also get babesiosis and anaplasmosis from tick bites and develop IMT as a reaction. Rocky Mountain spotted fever, which is caused by a tick-borne bacterium called Rickettsia rickettsii, is another possible cause.
Tularemia is one of the more common infections that can cause IMT and is not tick-borne. It is sometimes called rabbit fever because they are a common vector for it, but many mammal species can be carriers. Humans can also be affected by tularemia.